There is known a plating apparatus which is configured to hold a wafer, such as a wafer, and immerse the substrate in a plating solution held in a plating tank (see Patent literature 1 and Patent literature 2). As shown in FIG. 55, a substrate holder includes a plurality of inner contacts 100 which are brought into contact with a periphery of a substrate W, and a plurality of outer contacts 101 which are coupled to the inner contacts 100, respectively. Wires 104 that interconnect the inner contacts 100 and the outer contacts 101 are disposed in the substrate holder. When the substrate W is disposed at a predetermined position in the plating tank, the outer contacts 101 are brought into contact with a feeding terminal 103 which is coupled to a power source 102. An electric current is passed through the outer contacts 101 and the inner contacts 100 to the substrate W, so that a metal film is deposited on a surface of the substrate W in the presence of the plating solution.
Conventionally, an electrical resistance between the outer contacts 101 is measured while the substrate W is held on the substrate holder, before plating of the substrate W. The purpose of this is to detect a defect of a conductive layer, such as a seed layer, formed on the surface of the substrate W or a defect of the inner contact 100. Specifically, if a value of an electrical resistance between a certain pair of outer contacts 101 is extremely larger or smaller than a value of an electrical resistance between other pair of outer contacts 101, it can be judged that there is a defect in the conductive layer and/or the inner contacts 100. Therefore, it is possible to detect in advance a failure of plating due to the defect of the conductive layer and/or the inner contact 100 without actually performing the plating.
If the feeding terminal 103 and the outer contacts 101 are in a poor contact, the electrical resistance between the feeding terminal 103 and the outer contacts 101 may change. As a result, non-uniform current may flow through the outer contacts 101 to the inner contacts 100. In particular, a thickness of a conductive layer tends to be thin in recent years. Moreover, there is also a tendency to increase a density of the current to be passed to a substrate W. Therefore, even a slight variation in the electrical resistance between the outer contacts 101 may drastically hinder a uniformity of a thickness of a metal film formed on a substrate W. In order to solve such problem, the outer contacts 101 may be formed by an integral member. However, in this case, the electrical resistance between each pair of outer contacts 101 cannot be measured before plating of a substrate W.